Page 23 - CH6 (1)

Smaller peak: Satellite DNA
● Satellites are of two types—micro­satellites and mini satellites,

depending upon the base composition, length of segment and the
number of repetitive units.

● Satellites do not code for proteins, but have a major role to play in
DNA fingerprinting.

● Polymorphism is actually a result of mutation. A germ cell mutation
(which can pass on to the next generation through sexual
reproduction) gives rise to polymorphism in populations.

● In other words, an inheritable mutation if observed in higher
frequencies in a population is known as polymorphism.

● Polymorphisms arise normally in non­coding sequences because
mutations in non­coding sequences do not affect an individual’s

reproductive ability.

Methodology of DNA fingerprinting

● VNTR (variable number of tandem repeats) are satellite DNAs that
show high degree of polymorphism.

● VNTRs are used as probes in DNA fingerprinting.
● First of all, DNA from an individual is isolated and cut with restriction

endonucleases.
● Fragments are separated according to their size and molecular weight

on gel electrophoresis.
● Fragments separated on electrophoresis gel are blotted (immobilised)
on a synthetic membrane such as nylon or nitrocellulose.

● Immobilised fragments are hybridised with a VNTR probe.
● Hybridised DNA fragments can be detected by autoradiography.

● VNTRs vary in size from 0.1 to 20 kb.
● Hence, in the autoradiogram, band of different sizes will be obtained.
● These bands are characteristic for an individual. They are different in

each individual, except identical twins.